1. Field of the Invention
The present invention relates generally to specimen inspection and review. More particularly, the present invention relates to electron beam inspection and review systems.
2. Description of the Background Art
Automated inspection and review systems are important in process control and yield management for the semiconductor and related microelectronics industries. Such systems include optical and electron beam (e-beam) based systems.
In the manufacture of semiconductor devices, detection of electrical failure earlier in the fabrication process is becoming Increasingly important to shorten product development cycles and increase product yield and productivity. Advanced wafer inspection systems based on scanning electron microscopy technology has been used to detect electrical failure in-line as voltage contrast defects. However, as device design rules further shrink, and new processes (such as, for example, high aspect ratio (HAR) contacts in front-end-of-line (FEOL), HAR vias in back-end-of-line (BEOL), and dual damascene copper processes) are being widely implemented, it becomes more challenging to detect voltage contrast defects in ever increasing high aspect ratio device structures. Further, image contrast variation caused by uneven charge distribution can make e-beam inspection unstable or un-inspectable. Such contrast variation can occur from inside a die, from die to die, row to row, or wafer to wafer. In order to successfully inspect a wafer, control of surface charge is advantageous to 1) detect voltage contrast defects effectively, and 2) reduce image contrast variation during inspection.
FIG. 1 is a simplified diagrammatic representation of a conventional scanning electron microscopy configuration 100. As shown, a beam of electrons 102 is scanned over a sample 104 (e.g., a semiconductor wafer). Multiple raster scans 112 are typically performed over a small area 114 of the sample 104. The beam of electrons 102 either interact with the sample and cause an emission of secondary electrons 106 or bounce off the sample as backscattered electrons 106. The secondary electrons and/or backscattered electrons 106 are then detected by a detector 108 that is coupled with a computer system 110. The computer system 110 generates an image that is stored and/or displayed on the computer system 110.
Typically a certain amount of charge is required to provide a satisfactory image. This quantity of charge helps bring out the contrast features of the sample 104. Although conventional electron microscopy systems and techniques typically produce images having an adequate level of quality under some conditions, they produce poor quality images of the sample for some applications. For example, on a sample 104 made of a substantially insulative material (e.g., silicon dioxide), performing one or more scans over a small area causes the sample to accumulate excess positive or negative charge in the small area relative to the rest of the sample 104. The excess charge generates a potential barrier for some of the secondary electrons, and this potential barrier inhibits some of the secondary electrons from reaching the detector 108. Since this excess positive charge is likely to cause a significantly smaller amount of secondary electrons to reach the detector 108, an image of the small area is likely to appear dark, thus obscuring image features within that small area. Alternatively, excess negative charge build up on the sample can increase the collection of secondary electrons causing the image to saturate. In some cases, a small amount of charging is desirable since it can enhance certain image features (by way of voltage contrast) as long as it does not cause image saturation.
The excess charge remaining from a-previous viewing or processing may therefore cause distortion. One solution used in SEM devices is to flood the sample with charged particles from a separate flood gun at a time separate from the inspection. This flooding equalizes the charge appearing across the sample 104, thus improving the voltage contrast images. One drawback to this flooding procedure is the need to move the stage including the entire sample to the area of the flood gun. In order to accomplish the flooding, the inspection must stop to permit movement of the sample 104 to the area of the flood gun. This dramatically increases the overall time required for the inspection since movement and flooding of the sample may take ten minutes or more to complete. This produces an equally dramatic decrease in the throughput for the inspection process. Typically a full inspection of a sample 104 will require hundreds of scan lines across the sample and the dissipation of charge may be required after only a few scan lines have been completed. The total time required for a sample 104 to be inspected therefore is the sum of the separate intervals for charge dissipation (or precharging) and inspection.
In regards to the focus of an electron image, a change in the surface charge for the area being imaged can also cause the image to go out of focus. In addition, a change in the height of the area being imaged may cause the image to go out of focus. Existing techniques to deal with these variations in surface charge and sample height include measuring surface charge with a Kelvin probe or secondary electron cut-off points and measuring the sample height by way of light or capacitative sensors. The data from these measurements may then be used to determine an adjustment of the focus. However, these existing techniques are disadvantageously complicated and/or inefficient. For example, measurement of surface charge with a Kelvin probe involves a large area to make the measurement and is typically slow.
Hence, as discussed above, efficient and effective control over the charge on the surface of a sample 104 is desirable to improve the speed of obtaining images and the quality of images obtained during electron beam inspection or review. Furthermore, it is desirable to improve techniques for focusing an electron image in dependence on surface charge and sample height variations.